Chair base



FIG. 1 is a perspective view showing the top, opposite sides and frontof a chair base which forms my new design, the broken lines of a chairand chair base parts being for illustrative purposes only and forming nopart of my design;

FIG. 2 is a top plan view thereof;

FIG. 3 is a front elevational view thereof;

FIG. 4 is a rear elevational view thereof;

FIG. 5 is a left side elevational view thereof; and,

FIG. 6 is a right side view thereof.

The ornamental design for a chair base, as shown and described.